1. Field of the Invention
The invention relates to a ferroelectric transistor having two source/drain regions, a channel region and a gate electrode, a layer made of ferroelectric material being provided between the gate electrode and the channel region. A change in the conductivity of the transistor is dependent on the polarization state of the layer made of ferroelectric material. Ferroelectric transistors of this type are being investigated with regard to nonvolatile memories. In this case, two different logic values of a digital information item are assigned two different polarization states of the layer made of ferroelectric material. Further possible uses for ferroelectric transistors of this type are neural networks, for example.
The problem arises with these ferroelectric transistors that ferroelectric material disposed on the surface of a semiconductor substrate exhibits poor interface properties. Furthermore, diffusion processes occur between individual constituents of the ferroelectric material and the semiconductor substrate. In order to reduce the influence of these effects on the electrical properties of a ferroelectric transistor, it has been proposed to use, between the ferroelectric layer and the semiconductor substrate, an intermediate layer made of SiO2, (see European Patent EP 0 566 585 B1) or made of CeO2, Y2O3 or ZrO2 (see, for example, the reference by T. Hirai et al., Jpn, J. Appl. Phys. Vol. 36 (1997) pages 5908 to 5911 or H. Nyung Lee et al, Ext. Abst. Int. Conf. SSDM, Hamamatsu, 1997, pages 382 to 383). These materials are insulating stable oxides which ensure a sufficiently good interface between the ferroelectric layer and the surface of the semiconductor substrate.
In the ferroelectric transistor, the intermediate layer acts as an additional capacitance which, when a voltage is applied between the gate electrode and the semiconductor substrate, reduces that part of the voltage which is dropped across the ferroelectric layer. This impairs the punch-through of a voltage present at the gate electrode to the channel region. When storing information in the ferroelectric transistor by applying a sufficiently high voltage to the ferroelectric layer, so that the polarization of the ferroelectric material is changed, only part of the applied voltage is thus dropped across the ferroelectric layer.
It is accordingly an object of the invention to provide a ferroelectric transistor and a method for fabricating it which overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, in which the punch-through of a voltage present at the gate electrode is improved compared with known solutions, without impairing the interfaces.
With the foregoing and other objects in view there is provided, in accordance with the invention, a ferroelectric transistor. The ferroelectric transistor contains a semiconductor substrate having a surface, two source/drain regions disposed in the semiconductor substrate and each having a surface, a channel region disposed between the two source/drain regions in the semiconductor substrate and having a surface, and a metallic intermediate layer disposed on the surface of the channel region and forms a Schottky diode with the semiconductor substrate. The metallic intermediate layer also has a surface. A ferroelectric layer is disposed on the surface of the metallic intermediate layer and the ferroelectric layer has a surface. A gate electrode is disposed on the surface of the ferroelectric layer.
The ferroelectric transistor has the two source/drain regions and the channel region disposed in between in the semiconductor substrate. The metallic intermediate layer is disposed on the surface of the channel region and forms a Schottky diode with the semiconductor substrate. The ferroelectric layer is disposed on the surface of the metallic intermediate layer, the gate electrode being disposed on the surface of the ferroelectric layer. The ferroelectric transistor thus has the structure of a MESFET whose gate electrode is isolated from the actual metal semiconductor contact by the ferroelectric insulator.
The memory cell can be operated for example as now described. In order to read out the information, a short voltage pulse (for example a few ns) is applied to the gate electrode of the transistor with a direction such that the Schottky contact is operated in the reverse direction. The magnitude and duration of the voltage pulse is chosen such that the pulse suffices to reverse the polarization of the ferroelectric layer (duration of the order of magnitude of ns) but, does not suffice to allow the charge xcex94Q corresponding to the polarization-reversal operation or the charge Q corresponding to the dielectric proportion to flow through the Schottky contact operated in the reverse direction.
The effect that can thereby be achieved is that, during the pulse duration, part of the voltage present at the gate electrode is dropped across the Schottky contact.
When a voltage pulse is applied, two cases are to be decided.
In the first case, if the polarization direction of the ferroelectric layer at the beginning of the voltage pulse is such that it is rotated by the voltage pulse, then the voltage between the gate electrode and the semiconductor will initially drop, namely for the duration of the polarization-reversal operation, and only then be constant.
In the second case, if, by contrast, at the beginning of the voltage pulse at the gate electrode, the ferroelectric layer is already polarized in the direction of the voltage, then the ferroelectric layer behaves similarly to a purely dielectric layer, that is to say the voltage dropped between the gate electrode and the semiconductor during the pulse duration xcex94t is constant.
If the doping in the channel region of the transistor is chosen accordingly, then it is possible, using that part of the voltage at the gate electrode which is dropped across the Schottky contact, to control the transistor, that is to say to switch the transistor on or off, depending on whether a normally off or a normally on transistor is involved.
Thus, with a normally off transistor, the quantity of charge which flows through the channel of the transistor during the pulse duration will be smaller in the first case than in the second case. The opposite is true for a normally on transistor. By integrating the quantity of charge during the pulse duration, it is possible to evaluate the stored information.
If the written information was destroyed by the read-out operation (first case), then it must subsequently be written back to the cell.
The writing or erasing of information to or in a cell can be effected by a larger voltage between the gate electrode and the semiconductor than for the read-out of information, which brings about larger currents through the Schottky contact (both in the forward direction and in the reverse direction) of the configuration, with the result that the voltage dropped across it becomes zero after a short time and the entire voltage between the electrode and the semiconductor or the metal layer (the semiconductor and metal layer are at the same potential) is dropped across the ferroelectric layer.
As an alternative, the writing or erasing of information can also be effected on a different, longer time scale than for the read-out, with the result that, during longer pulse durations, more charge can flow through the Schottky contact and, therefore, voltage is no longer dropped across the Schottky contact after a short time (for example tens of ns) in this case as well.
Further methods of operating the memory cell are possible.
Consequently, in the ferroelectric transistor, the additional capacitance between semiconductor substrate and ferroelectric layer that is present in the known ferroelectric transistors is obviated both when storing information and when erasing information. At the same time, direct contact between the ferroelectric layer and the semiconductor surface is avoided.
It lies within the scope of the invention to provide the metallic intermediate layer to be made of Pt, WSi2, Au or Ti. The use of WSi2 has the advantage that WSi2 can be produced with good interface properties on the surface of a semiconductor substrate.
Preferably, the surface of the channel region terminates with the surface of the semiconductor substrate, while the surface of the source/drain regions is disposed below the surface of the semiconductor substrate. This avoids a short circuit between the source/drain regions via the metallic intermediate layer.
As an alternative, such a short circuit can also be prevented by the source/drain regions having a lateral doping profile, so that they form a Schottky contact in the region of overlap with the metallic intermediate layer. In this refinement, it is not necessary to lower the surface of the source/drain regions in order to avoid the short circuit.
The use of platinum for the metallic intermediate layer has the advantage that many ferroelectric materials are very well suited to the deposition on platinum. Such deposition methods are being investigated and optimized in connection with the development of ferroelectric capacitors.
The intermediate layer is preferably composed of two partial layers, a first partial layer adjacent to the surface of the channel region being formed from WSi2 and a second partial layer adjoining the ferroelectric layer being made from platinum. As a result, a good interface with the semiconductor substrate and good deposition conditions for the ferroelectric layer are obtained at the same time.
All ferroelectric materials are suitable for the ferroelectric layer. In particular, the ferroelectric layer has strontium bismuth tantalate (SBT), lead zirconium titanate (PZT), lithium niobate (LiNbO3) or barium strontium titanate (BST)
With regard to the dielectric strength of the ferroelectric transistor, it is advantageous to provide the metallic intermediate layer, the ferroelectric layer and the gate electrode with common sidewalls provided with insulating spacers. In this case, it is advantageous to form the insulating spacers from ferroelectric material, since this avoids impairment of the properties of the ferroelectric layer by an interface with another material.
Such a ferroelectric transistor is preferably fabricated within silicon process technology. In particular, a monocrystalline silicon wafer, a silicon-on-insulator (SOI) substrate or an SIC substrate is suitable as the semiconductor substrate.
The gate electrode is formed from platinum or doped polysilicon, and the semiconductor substrate contains silicon.
Furthermore, a III-V semiconductor substrate, for example a GaAs substrate, in particular made of semi-insulating GaAs, is suitable as the semiconductor substrate. Since MESFETs are often realized in semi-insulating GaAs substrates, the memory cell can readily be integrated into this technology.
In order to fabricate the ferroelectric transistor, the metallic intermediate layer, which forms the Schottky diode with the semiconductor substrate, the ferroelectric layer and the gate electrode are formed by deposition and patterning. Source/drain regions are formed in the semiconductor substrate on two mutually opposite sides of the gate electrode. The metallic intermediate layer can be patterned both by lift-off technology and by etching technology. The ferroelectric layer can be formed by a single-stage or multistage sol gel method or by deposition in a chemical vapor deposition (CVD) process and subsequent heat treatment.
In accordance with a concomitant feature of the invention, the source/drain regions are formed with a lightly doped drain profile.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a ferroelectric transistor and a method for fabricating it, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.